Starbucks makes the mugs themselves, and they have a pretty unlimited supply. They are collectable mugs found throughout the world now at city-specific Starbucks. My mom has been collecting them, so that scalper scum (I'm sorry, but I hate anybody who profits on lessening a limited supply to those who actually want those items and then reselling it to them at exponentially higher prices) is profiting on those who aren't able to make it to USH to buy the mug but wants to complete their collection. Luckily, this isn't a case where nobody else will be able to get them.
Edit: actually, UOR just ran out of stock so apparently it's possible and Starbucks is not stopping it. What a shame